1. Field of the Invention
Cellular membranes are dynamic structures, where proteins are frequently capable of moving in the plane of the membrane, but remain non-covalently bound to the membrane. The membrane proteins serve a wide variety of functions, providing for host recognition, processing of polypeptides, transport of inorganic and organic compounds across the membrane barrier, ion and energy pumps, and the like. The membrane proteins are essential for the viability of the cell.
The membrane proteins which are external to the cell can frequently act as receptors, binding to a wide variety of ligands. Such receptors can be involved in the predisposition to disease, host specificity for viral infection or immunity, activation of intracellular enzymes, or other essential cellular function.
The plasma membrane proteins assume a variety spatial relationships with the membranes. The protein can extend from either or both faces of the membrane, extending into the cytoplasm or into the external medium. The membrane protein may extend only once through the membrane or extend a multiplicity of time through the membrane.
One of the mysteries which is presently being unraveled is how a cell designates the location to which a particular polypeptide product is to be directed. The ability to direct particular polypeptides to a particular cellular site can have far ranging consequences for the cellular production of a wide variety of organic products of interest, for use in the diagnosis and treatment of disease, and for the understanding of cellular functions and responses to various ligands.
2. Description of the Prior Art
Anderson et al., J. Cell Biology (1982) 93:501 report that both secretory and integral transmembrane protein (ITMP) signal sequences are recognized by common signal recognition elements. Blobel, Proc. Nat.. Acad. Sci. USA (1980) 77:1496 postulates a mechanism for intracellular protein topogenesis. Early et. al., Cell (1980) 20:313; Rogers et al. Cell (1980) 20:303; as well as others, describe the DNA and protein sequences and genomic DNA exon structure of the IgM heavy chain. Lingappa et al., Proc. Natl. Acad. Sci. USA (1977) 74:2432 describe a cell-free transcription-linked translocation-coupled translation system with native secretory protein mRNAs. Katz et al., ibid. (1977) 74:3278 describe the same system for membrane mRNAs. Lodish et al., Int. Rev. Cytol. Supp. (1981) 12:247 postulated that the extreme carboxy terminus of an ITMP serves as a passive "anchor" to retain an already completed chain in the membrane. Engleman and Steitz, Cell(1981) 23:411-422 hypothesized that translocation was mediated exclusively by the hydrophobic-polar helical hairpin of a nascent chain interacting with the lipid bilayer. The plasmid pU6 is described by Rogers et al, Cell (1980) 20:303. The use of dog pancreas microsomal membranes with the cell-free system described above is described by Muller et al., J. Biol. Chem. (1982) 257:11860-11863. Fusion proteins between secretory and cytosolic proteins are described by Moreno et al., Nature (1980) 286:356. Conversion of ITMPs into secretory proteins by carrying out deletions of carboxy transmembrane segments is described by Boeke and Model, Proc. Natl. Acad. Sci. USA. (1982) 79:5200; Gething and Sambrook, Nature (1982) 300:598; Kondor-Koch et al.. Proc. Natl. Acad. Sci. USA (1982) 79:4525 and Rose and Bergmann, Cell (1982) 30:753. Perara and Lingappa (1980) J. Cell Biology, 101:2292-2301 and Perara, et al. (1986) Science 232:348-352 describe positional varying of a signal sequence and the effect of structural gene termination on peptide translocation, which disclosures are incorporated herein by reference.